Signal conditioner for recovering dominant signals from swirl-type meters

ABSTRACT

A signal conditioner adapted to extract the dominant frequency from the composite output signal of a swirl or vortex type flowmeter and to exclude low and high frequency noise components whereby by measuring only the dominant frequency, one obtains an accurate reading of fluid flow quantity. The signal conditioner includes a square-wave-generating trigger to generate the measuring frequency, which trigger undergoes a change in output state only when the amplitude of the signal applied thereto rises above a predetermined high level and reverts to its original output state when the amplitude of the applied signal falls below a predetermined lower level, no change in state occurring with respect to amplitude fluctuations lying within the window defined by the two levels. The composite signal is applied to the trigger through an automatic gain control circuit whose attenuation is automatically changed as a function of frequency whereby the amplitude of the composite signal applied to the trigger is greatest when the dominant frequency thereof is at the high end of the operating range, thereby effectively broadening the window for low operating frequencies and narrowing the window for high operating frequencies to effect low-frequency noise rejection at high operating frequencies and high-frequency noise rejection at low operating frequencies.

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United States Patent [191 [111 3,709,034 Herzl [451 Jan. 9, 1973 54]SIGNAL CONDITIONER FOR 57 ABSTRACT RECOVERING DOMINANT SIGNALS A signalconditioner adapted to extract the dominant FROM swIRL- METERS frequencyfrom the composite output signal of a swirl or vortex type flowmeter andto exclude low and high frequency noise components whereby by measuringonly the dominant frequency, one obtains an accurate [73] Assignee:Fischer 8: Porter Company, Warreading of fluid flow quantity. The signalconditioner minster, Pa. includes a square-wave-generating trigger togenerate the measuring frequency, which trigger undergoes a [22] Filed:Feb- 2, 19 1 change in output state only when the amplitude of thesignal applied thereto rises above a predetermined [21] Appl. No.:111,913 high level and reverts to its original output state when theamplitude of the applied signal falls below a 52 us. Cl ..73 194 B,307/233, 324/77 B, predetermined level change 324/78 R, 328/140 ringwith respect to amplitude fluctuations lying within 51 Int. Cl ..G01s1/00 h Wind dhhhed by the levels- The whphhite 58 Field ofSearch.....73/194 B; 307/233, 235, 295; slghal is PP E the trigger f han ahwlhahc 324/78 R, 77 328/140, 165 gain control circuit whoseattenuation is automatically changed as a function of frequency wherebythe amplitude of the composite signal applied to the trigger is [75]Inventor: Peter J. Herzl, Morrisville, Pa.

[56] References Cited greatest when the dominant frequency thereof is atthe UNITED STATES PATENTS high end of the operating range, therebyeffectively broadening the window for low operating frequencies3,522,449 8/1970 McMurtrie ..307/235 and narrowing the window foroperating frequen 3,433,979 3/1969 Hubbard ..307/235 cies to effectlow-frequency noise rejection at high operating frequencies andhigh-frequency noise rejec- Primary Examiner ]ames Gm tion at lowoperating frequencies.

Assistant ExaminerHerbert Goldstein Attorney-Michael Ebert 8 Claims, 8Drawing Figures SIGNAL CONDITIONER FOR RECOVERING DOMINANT SIGNALS FROMSWIRL-TYPE METERS BACKGROUND OF THE INVENTION This invention relatesgenerally to swirl-type flowmeters, and in particular to a signalconditioner adapted to extract the dominant signal from the output of aswirl-type flowmeter and to exclude noise and lesser signal components.

A new type of volumetric flowmeter is disclosed in the article ofRodely, et a1, entitled A Digital F-lowmeter Without Moving Parts,published in 1965 by the American Society of Mechanical Engineers (8April 1965 WA/FM6). This flowmeter, which is of the swirl type, is nowknown commercially under the trademark Swirlmeter. Meters of this typeare also described in U.S. Pat. Nos. 3,279,251; 3,314,289; and U.S. Pat.No. Re.26,410, among others.

In a Swirlmeter, a homogeneous fluid whose flow rate is to be measuredis forced to assume a swirl component by converting pressure energy intokinetic energy. This is accomplished by feeding the fluid into the inletsection of a flow tube having a fixed set of swirl blades thereinimparting a swirling motion to the fluid passing therethrough.Downstream of the swirl blades in the tube is a Venturi section whichfirst constricts and then expands the flow passage to transform theswirling motion into precessional movement in the expanding region ofthe Venturi section. Precession takes place about the central axis ofthe flow tube at a discrete frequency that is a function of thevolumetric flow rate. De-swirl blades in the outlet section of the flowtube serve to straighten out the fluid leaving the meter. Cyclicvariations in local fluid velocity, occurring by reason of precession,are detected to provide electrical pulses whose frequency is measured toprovide an indication of flow rate.

In a commercial form of Swirlmeter manufactured by Fischer & PorterCompany of Warminster, Pennsylvania, the assignee herein, the vortexprecession in the meter is sensed by a probe having a thermistor at itstip which is caused to operate in its self-heat region by applying aconstant current across it, heating the thermistor to a temperatureabove that of the passing fluid. Because of the periodic cooling actionproduced by the vortex precession, the change in thermistor resistanceproduces concurrent voltage variations.

These voltage variations are detected, amplified and shaped by adetector-amplifier associated with the thermistor.

Included in the detector-amplifier is a frequencycompensating amplifierthat compensates for the frequency response of the thermistor.lnherently, the thermistor output is highest at low frequencies anddecreases with increasing frequency. The frequencycompensating amplifieradjusts for the amplitude rolloff characteristic of the thermistor. ThisSwirlmeter is described in the Instruction Bulletin for Series 108 1000Swirlmeter, published by Fischer & Porter Co.

Though the variable of interest in the Swirlmeter system is the flowrate of the fluid being measured, the system responds to more than onevariable. The frequencies reflecting the other variables aresuperimposed on the fundamental system frequency to produce a compositesignal. These frequency components may be derived from blower orcompressor. pulsations, or hydraulic noise. In addition, the amplitudeof the process variable signal from the detector-amplifier may vary as aresult of still other variables. Some of these variables are related tofrequency and may be compensated for by having different levels ofamplifier gain at different frequencies, whereas other variablesunrelated to frequency result in amplitude variations. 1n the case of aSwirlmeter, such non-related variables are temperature, pressure, gasdensity, meter size, etc. The irrelevant frequencies in the Swirlmeteroutput, unless discriminated against, are a primary source ofmeasurement error.

In order to separate the dominant frequency representing flow rate fromall noise and irrelevant frequencies found in the composite processvariable signal, the Fischer & Porter Swirlmeter described in theabove-mentioned Instruction Bulletin makes use of a signal conditionerof the type described in the McMurtrie U.S. Pat. No. 3,522,449, whereinthe process variable signal is applied to a three-band filter network.Each band of this network is designed to pass only those signals withina particular frequency span while sharply attenuating those frequenciesoutside its specific range. The output signal from each of the threenetworks is introduced simultaneously to one of three separate amplifierchannels.

These channels (low, middle and high) include a minimum signal detectorfunctioning to enable signal transmission from only one of the threechannels at any one time, whereby the signal conditioner willautomatically select the frequency band containing the dominant signalvoltage related to flow rate and will direct the signal within the bandto its own output.

Though the signal conditioner in present use functions effectively, itis relatively complex and expensive and adds substantially to theover-all cost of the Swirl meter system.

SUMMARY OF THE INVENTION In view of the foregoing, it is the main objectof this invention to provide a signal conditioner for use in conjunctionwith a vortex-type meter for automatically extracting the dominantsignal from a composite signal within a given frequency range and forrejecting all other components.

Also an object of the invention is to provide a relatively simple andinexpensive signal conditioner to extract the dominant signal from acomposite Swirlmeter signal and to generate a periodic square-wave whosemeasurement affords an accurate index to flow rate.

Though the invention will be described in conjunction with a Swirlmeter,it is also usable with other systems requiring discrimination between adominant frequency and other, non-relevant frequencies present in acomposite output signal, such as in a vortex-type flowmeter of the typedescribed in the co-pending application of Burgess, Ser. No. 855,153,filed Sept. 21, 1969, now U.S. Pat. No. 3,589,185.

Briefly stated, these objects are attained in a signal conditionerincluding a trigger which undergoes a change in output state only whenthe amplitude of signal applied thereto exceeds a predetermined highlevel and reverts to an original output state when the amplitude fallsbelow a predetermined lower level, the

trigger being insensitive to amplitude changes lying within the windowdefined by the two levels.

The composite signal derived from a swirl-type or vortex meter isapplied to the trigger through an automatic gain control circuit in theform of a variable attenuator which acts automatically to decrease itsattenuation as the dominant frequency of the applied signal increases,thereby effectively broadening the window for low operating frequencieswithin the operating range of the system to effect low-frequency noiserejection at high operating frequencies and effectively narrowing thewindow for low operating frequencies to effect highfrequency noiserejection at such low frequencies.

OUTLINE OF THE DRAWING For a better understanding of the invention aswell as other objects and further features thereof, reference is made tothe following detailed description to be read in conjunction with theaccompanying drawing, wherein:

FIG. 1 is a simplified block diagram ofa signal conditioner inaccordance with the invention, operating in conjunction with aSwirlmeter system;

FIG. 2 illustrates the operation of the trigger circuit included in thesystem;

FIGS. 3A, 3B and 3C are wave forms illustrating the effect of theautomatic gain control circuit on the voltage applied to the trigger;

FIG. 4 shows the output wave form of the trigger for a dominantlow-frequency input;

FIG. 5 shows the output wave form of the trigger for a dominanthigh-frequency input; and

FIG. 6 is a schematic diagram of the signal conditioner.

DETAILED DESCRIPTION OF THE INVENTION Referring now to FIG. 1, there isshown a Swirlmeter primary 10 utilizing the principle of vortexprecession to develop a process-variable signal frequency that isdirectly related and proportional to flow velocity. Mounted on the bodyof meter 10 in the area where the vortex precession reaches the innerdiameter of the meter wall is a sensor probe having a thermistor 11 inits top. Thermistor 11 is coupled to a detector-amplifier 12 mounted onthe Swirlmeter primary. Detector-amplifier 12 incorporatesfrequency-compensation means to compensate for the frequency response ofthe thermistor. In practice, other forms of sensors may be used.

The signal conditioner, whose first stage is an automatic gain control(AGC) device 13, receives from detector-amplifier 12 a composite signalin an operating frequency range, which in practice, as indicated bycurve 14, may extend between lOHz and 1000Hz. In addition to thedominant or primary signal representing the flow variable of interest,the composite signal in the output of detector-amplifier 12, asrepresented by wave form 15, has a large noise content constituted bylow and high frequency components other than the primary signal.

In practice, the A-C content of the composite signal may be in anamplitude range of 0.1 VPP (0.035 VRMS) to 5 VPP (1.8 VRMS). The task ofthe signal conditioner in accordance with the invention is to improvethe signal-to-noise ratio of the incoming signal and to retransmit a 15VPP square wave, representing the basic frequency, to a frequency meter16 calibrated in terms of flow rate. By rejecting all frequencies otherthan the dominant frequency, the signal conditioner renders the systemhighly accurate.

Before describing the function of the elements constituting the signalconditioner and the manner in which they operate, we shall identifythese elements. The output of the automatic gain control device 13,which is the input stage, is coupled through a tracking or slidinghigh-pass filter 17 to an amplifier 18 whose output is applied to atrigger 19, and is also fed back by way of a frequency-shaping circuit20 to the AGC device 13. square-wave pulses produced by trigger 19 go toa driver output stage 21 and are also fed back to a one-shot circuit 22whose pulses are applied through filter control network 23 to trackinghigh-pass filter 17.

The heart of the signal conditioner is trigger 19, whose function is toturn a noisy composite signal into a square wave representing thefundamental frequency. In practice, this circuit may be a Schmitttrigger provided with a predetermined trigger differential. As shown inFIG. 2, the trigger is designed to change its output state from state tostate at a predetermined high voltage level L, and to revert back to itsoriginal state at a somewhat lower voltage level L The differencebetween levels L, and L constitutes the trigger window W, within whichno change in voltage in any way affects the trigger output.

Thus, with a composite signal CS having a dominant fundamental frequencyand low and high frequency components, when the signal applied to thetrigger exceeds level L at intersection point X, its state switches fromto No change in state occurs until the amplitude of signal CS drops tothe intersection point Y in level L at which point the trigger revertsto state and remains in that state until the amplitude of signal CSagain reaches level L Thus the trigger generates a square wave whoseperiodicity corresponds to the dominant frequency of the signals.

The width of trigger window W is fixed. However, in order to improvelow-frequency noise rejection at high operating frequencies, andhigh-frequency noise rejection at low operating frequencies, it becomesnecessary to vary the apparent size of the trigger window so that with alow-frequency fundamental, the window appears to be broad, causing thetrigger to discriminate against the high-frequency components imposed onthe fundamental, whereas with a high-frequency fundamental, the windowis narrowed to exclude the low-frequency components in the compositesignal.

FIGS. 3A, 3B and 3C illustrate typical wave shapes in a noisy systemwhere band-width is limited to a given operating range by roll-offfilters at both ends of the band, as is accomplished indetector-amplifier 12 (FIG. 1). Thus, as shown in FIG. 3A, there is aroll-off below IOHz, the lower end of the range, and above IOOOHz, theupper end thereof.

Low-frequency composite signals have a considerable high-frequencyactive band-width above the operating frequency in the range. There is,however, little low-frequency noise since the system is then operatingnear the low-frequency cut-off point of lOHz. Hence, with an operatingsignal of l5Hz, most of the lowfrequency noise is below the activeband-width of the system.

Conversely, a high-frequency composite signal contains mainlylow-frequency noise, since most of the active band width lies below it.Hence, with an operating signal of, say, 905 Hz, most of thehigh-frequency noise is above the active band width of the operatingrange which cuts off at 1000 Hz.

In order to vary the apparent width of window W, the amplitude of thecomposite signal applied to the trigger is automatically increased as afunction of frequency by the gain control device 13, to be laterdescribed, so that at the low end of the operating range of the system,which, in the sample given by FIG. 3A, is Hz, the amplitude of thesignal is relatively low, whereas at the high end, which is 1000 Hz, theamplitude is high.

Thus a typical composite signal CS applied to the trigger, having alow-frequency fundamental, as shown in FIG. 3B, is of relatively lowamplitude, and while the width of window W is fixed, the window, whichin this instance is about 70 percent of the peak-to-peak amplitudedimension of the applied signal, appears large with respect to thecomposite signal and discriminates against the high-frequency noisecomponents. Thus the square-wave trigger output, as shown in FIG. 4,corresponds to the low-frequency fundamental and disregards all othercomponents in the composite signal.

However, a typical composite signal having a highfrequency fundamentalfrequency, as shown in FIG. 3c, is of relatively high amplitude. Whilethe width of the trigger window W is still fixed, the window now isabout 23 percent of the peak-to-peak amplitude dimension of the appliedsignal and hence appears narrow with respect thereto and discriminatesagainst the low frequency noise components. Thus, in this instance, thesquare-wave trigger output, as shown in FIG. 5, corresponds to thehigh-frequency fundamental and disregards all other components.

Returning now to FIG. 1, the manner in which the composite signalapplied to trigger 19 is automatically varied in amplitude to produceapparent changes in window size will now be explained.

The composite signal of the Swirlmeter detector-amplifier 12 appears atpoint A and its wave form is represented by form 15. This signal entersthe automatic gain control (AGC) stage 13 which functions to regulatethe amplitude at point B, the input to the trigger 19. In practice theAGC is an attenuator circuit whose degree of attenuation is variedautomatically to provide a signal of approximately 6VPP at a fundamentalof 200 Hz and of approximately IOVPP at 1000 Hz, thereby raising thegain with increasing frequency.

The operation of the AGC circuit is controlled by frequency-shapingnetwork 20, a low-pass filter preferably in the form ofa simple R-Ccircuit consisting of one resistor and one capacitor coupled to point B.The output of the network is applied at point C to AGC circuit 13. Thefilter output appearing at point C has the waveform shown at 24, and itwill be seen that the gain is highest at the low-frequency end of therange and diminishes as frequency increases.

Since the resultant attenuation introduced by AGC circuit 13 depends onthe control voltage applied thereto by the frequency-shaping network 20,the attenuation is greatest at the low-frequency end of the range.Consequently, at point B, the amplitude of the composite signal appliedto the trigger is low for the low-frequency signal and high for thehigh-frequency signal.

Tracking high-pass filter 17 makes use of a single or multiple polefilter using variable resistance capacitance or inductive elements toshift the filter up or down in frequency, as indicated by graph 25, as afunction of voltage applied thereto by filter control network 23. Inpractice, filter 17 may be constituted by a two-pole R-C filter using FETs as adjustable resistance elements. As the gate voltage in the FETsis raised, they lower their resistance and the operating point of thefilter moves to a higher frequency, as indicated by the dashed lines ingraph 25.

The output from tracking filter 17 at point D, represented by wave form26, is amplified by filter amplifier 18, which is an operationalamplifier whose gainvs.-frequency characteristic is indicated by graph27. It will be seen that the gain-vs-frequency characteristics of theamplifier are set up to provide some degree of high-frequency roll-off.

The one-shot circuit 22 coupled to the output of trigger 19 at point E,produces a single pulse of constant height and width for each cycle ofoutput frequency, irrespective of the operating frequency. In practice,one-shot 22 is a temperature-compensated device producing, at point F, apulse of about 9V amplitude at I60 microseconds in width each time thepositive-going edge of the square wave of the trigger output appears.

Filter control network 23 converts pulses from oneshot 22 into a D-Cvoltage varying as a function of frequency suitable for controlling thegates of the FETs acting as variable resistors in tracking filter 17,thereby closing the feedback loop in the tracking filter. While thetrigger, in combination with the AGC, acts to remove a high degree ofnoise, tracking filter 17, in combination with amplifier 18,nevertheless has a beneficial effect, for it filters the signal fed tothe trigger circuit to optimize the fundamental frequency andtie-emphasize all other components, which is particularly useful with avery noisy signal.

Thus the wave form 28,- of the signal at point B, the input of trigger19, is that of an a-c signal in which the operating frequency ispredominant, whereas the wave form 28,, at point E, the trigger output,is a square-wave whose frequency corresponds to the dominant operatingfrequency.

Referring now to FIG. 6, the schematic circuit of a preferred embodimentof a signal conditioner in accordance with the invention is shown, thestages which correspond to the stages shown in FIG. 1 being identifiedby like reference numerals.

The input from detector-amplifier 12 (FIG. 1) is applied at inputterminal A leading to the AGC circuit 13 wherein gain control isachieved by varying the d-c current through diode 30. As currentincreases, the voltage on the base of transistor 31 is raised to cause adrop in the resistance of the diode, thereby increasing attenuation. Thefrequency characteristics of the AGC circuit are determined by capacitor32 and resistor 33 in the frequency-shaping network 20.

The output of the AGC circuit is applied to sliding high-pass filter 17through a buffer-amplifier 34 including an emitter-follower 35 havingunity gain to provide a low source impedance for the sliding filter. Inthe sliding high-pass filter 17, FETs 36 and 37 act as the adjustableresistance elements and as the gate voltage thereon, which is derivedfrom filter control network 23, is raised, the FETs lower theirresistance and the filter operating point moves to a higher frequency.

Filter amplifier 18 amplifies the output of sliding high-pass filter 17,the output of the amplifier at point B being fed back to resistor 33,the control input of the AGC circuit 13. The amplifier output is fed totrigger 19 through a zero-flow cut-off circuit 38 which includes atransistor 39.

The arrangement is such that when the output of filter amplifier 18drops below a predetermined mean value, transistor 39 grounds the signalapplied to the trigger. This arrests random noise at zero flow fromentering the trigger. Trigger 19 is an inverter whose positive swing isclamped to a predetermined voltage level (i.e., volts) by zener diode40. The output of driver 16 is a positive-going l5-volt square-wave inrelation to common.

One-shot 22 is a temperature-compensated device producing a pulse ofconstant height and duration (i.e., 9 volts l60 microseconds) every timethe positive going edge of the input square-wave turns on a transistor41. The filter control network 23 converts the pulses from the one-shotinto a voltage used to control the gates of FETs 36 and 37 in thesliding highpass filter 17.

While there have been shown and described several embodiments of signalconditioner for recovering dominant signals from swirl-type meters inaccordance with the invention, it will be understood that many changesand modifications may be made therein without, however, departing fromthe essential spirit of the invention.

1 claim:

1. A signal conditioner adapted to extract the dominant frequency fromthe composite output signal of a swirl or vortex type flowmeter and toexclude low and high frequency noise components therefrom so that bymeasuring the dominant frequency, an accurate reading is obtained offlow rate, said conditioner comprising:

A. a square-wavegenerating trigger having a differential characteristicwherein a change in output state takes place when the amplitude of thesignal applied thereto exceeds a predetermined high level, the triggerreverting to its original state when the amplitude of said signal fallsbelow a predetermined lower level, no change in state occurring withrespect to amplitude fluctuations lying within the fixed window definedby the two levels,

B. a voltage-responsive variable attenuator adapted to produceattenuation to a degree determined by an applied control voltage,

C. means including an amplifier to apply the composite signal from theflowmeter through the variable attenuator and said amplifier to theinput of the trigger, and

D. wave-shaping means coupled to the output of said amplifier to producea control voltage whose amplitude is a function of frequency and toapply said control voltage to said attenuator to cause the amplitude ofsaid compositev signal applied to said trigger to increase as thefrequency thereof increases in order to vary the apparent width of thefixed window, thereby effectively broadening the window for lowfrequencies and narrowing the window for high frequencies to effectlow-frequency noise rejection at high operating frequencies andhigh-frequency noise rejection at low operating fre uencies.

2. A con itioner as set forth in claim 1, further including a driverstage coupled to the output of said trigger to produce square-wavepulses of constant height and width for application to a frequency meterproviding a reading of flow rate.

3. A conditioner as set forth in claim 1, further including a trackinghigh-pass filter interposed between said attenuator and said amplifierto optimize the amplitude of the dominant frequency in said compositesignal.

4. A conditioner as set forth in claim 3, further including a one-shotcircuit coupled to the output of said trigger to produce a pulse forevery cycle of output frequency irrespective of the operating frequency,and a filter control network coupled to said one-shot circuit andresponsive to pulses produced thereby to produce a voltage whoseamplitude is a function of the rate of said pulses for controlling saidtracking filter.

5. A conditioner as set forth in claim 3, further in cluding a bufferamplifier interposed between said attenuator and said tracking filter.

6. A conditioner as set forth in claim 1, wherein said attenuatorincludes a diode whose resistance is varied to effect a varying degreeof attenuation.

7. A conditioner as set forth in claim 1, further including a zero flowcut-off circuit interoised between said amplifier and said triggercircuit, said zero flow cut-off circuit grounding the output of saidamplifier when the flow rate is substantially at zero level and therebypreventing random noise from entering said trigger circuit.

8. A conditioner as set forth in claim 1, wherein said trigger is aSchmitt circuit.

, UNITED STA ES PATENT QFFICE CERTIFICATE OF CORRECTION Petent No-3,709, 034 U I Dated January 1.97 3

PETER J. I-[ER'ZL Inyentor(e) It is certified that error appears in thefab ve-identified patent and that said Letters Patent are herebycorrected as shown below:

"Column 7 Line ll should have-read:

v -tering the trigger.' Trigger 19 is an amplifier with positivefeedback, The output driver 21 Signed and sealed this 22nd c1ey of May1973.

(SEAL).

Attest: I v

EDWARD M.PLETCHER,JRR. r I ROBERT GOTTSCHALK- Attesti'ng Officer ICommissioner of Patents FORM F'O-1050 (O-69] US COMM DQ' 50375-5 69 U.S.GOVERNMENT PRINHNG OFFICE: 1969 0-366-334

1. A signal conditioner adapted to extract the dominant frequency fromthe compositE output signal of a swirl or vortex type flowmeter and toexclude low and high frequency noise components therefrom so that bymeasuring the dominant frequency, an accurate reading is obtained offlow rate, said conditioner comprising: A. a square-wave-generatingtrigger having a differential characteristic wherein a change in outputstate takes place when the amplitude of the signal applied theretoexceeds a predetermined high level, the trigger reverting to itsoriginal state when the amplitude of said signal falls below apredetermined lower level, no change in state occurring with respect toamplitude fluctuations lying within the fixed window defined by the twolevels, B. a voltage-responsive variable attenuator adapted to produceattenuation to a degree determined by an applied control voltage, C.means including an amplifier to apply the composite signal from theflowmeter through the variable attenuator and said amplifier to theinput of the trigger, and D. wave-shaping means coupled to the output ofsaid amplifier to produce a control voltage whose amplitude is afunction of frequency and to apply said control voltage to saidattenuator to cause the amplitude of said composite signal applied tosaid trigger to increase as the frequency thereof increases in order tovary the apparent width of the fixed window, thereby effectivelybroadening the window for low frequencies and narrowing the window forhigh frequencies to effect lowfrequency noise rejection at highoperating frequencies and high-frequency noise rejection at lowoperating frequencies.
 2. A conditioner as set forth in claim 1, furtherincluding a driver stage coupled to the output of said trigger toproduce square-wave pulses of constant height and width for applicationto a frequency meter providing a reading of flow rate.
 3. A conditioneras set forth in claim 1, further including a tracking high-pass filterinterposed between said attenuator and said amplifier to optimize theamplitude of the dominant frequency in said composite signal.
 4. Aconditioner as set forth in claim 3, further including a one-shotcircuit coupled to the output of said trigger to produce a pulse forevery cycle of output frequency irrespective of the operating frequency,and a filter control network coupled to said one-shot circuit andresponsive to pulses produced thereby to produce a voltage whoseamplitude is a function of the rate of said pulses for controlling saidtracking filter.
 5. A conditioner as set forth in claim 3, furtherincluding a buffer amplifier interposed between said attenuator and saidtracking filter.
 6. A conditioner as set forth in claim 1, wherein saidattenuator includes a diode whose resistance is varied to effect avarying degree of attenuation.
 7. A conditioner as set forth in claim 1,further including a zero flow cut-off circuit interoised between saidamplifier and said trigger circuit, said zero flow cut-off circuitgrounding the output of said amplifier when the flow rate issubstantially at zero level and thereby preventing random noise fromentering said trigger circuit.
 8. A conditioner as set forth in claim 1,wherein said trigger is a Schmitt circuit.